Speech recognition is known in the art. Limited vocabulary speech recognizers operate by matching the incoming speech to a collection of reference speech models and selecting the reference model(s) which best match(es) the incoming speech. Limited vocabulary speech recognizers are used for speech dialing, in which the user says a phone number and the speech recognizer determines which digits were said and provides the recognized digits to the automatic dialing system of a telephone. “Digits” typically include the numerical digits, symbols, such as *, # and +, and pause and editing words such as “clear”, “cancel”, “dial” and save”. Speech dialers exist on cellular telephones to provide ‘hands-free dialing’ during driving.
Speech dialers, especially those operating in a car environment, often have difficulty determining the digits said, since some digits have similar sounding names in certain languages. To improve recognition performance, some speech recognition systems add constraints to the recognition process, based on the natural constraints of the dialing process.
For speech dialing, if the user defines the country where the phone is used, then the “numbering plan” of that country may be used to constrain at least some of the digits. For example, the numbering plan of the United States states that the first number of an area code may not be a 0 or a 1. Furthermore, all area codes are comprised of three digits, all exchanges are comprised of three digits, and there are four remaining digits. A more complete numbering plan for the US is listed below, where N is a digit from 2-9 and X is a digit from 0-9 and ‘-’ indicates an end of a phrase:
1 digit number: 0 (operator)
3 digit number: N11
3 digit number: *XX
4 digit number: *XXX
7 digit number: NXX-XXXX
10 digit number: NXX-NXX-XXXX
11 digit number: 1-NXX-NXX-XXXX
11 digit number: 0-NXX-NXX-XXXX
If the user says three digits, then the speech dialer, using the numbering plan, can ‘guess’ that the first digit was either a star (*) or an N. Similarly, if seven digits were said, then the first digit cannot be a zero or a one. This slight constraining of the speech recognizer significantly improves the recognition results. In addition to the hard constraints described above, speech recognizers sometimes apply soft constraints, i.e. all digit sequences are allowed but prior probabilities are used to elevate the probabilities of recognizing certain sequences and reduce the probabilities of others.
In 2003, the websites of the following entities included in them descriptions of various numbering plans:
World Telephone Numbering Guide
North American Numbering Plan Administration
Vertical Service Codes (dialing numbers specific to carriers)
It will be appreciated that, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.